The present invention relates in general to control activities in a situation where a cellular radio system terminal is simultaneously in radio connection with at least two base stations. In particular, the invention relates to the transmission of parameters related to call control between the cellular radio system parts which data transmission in such a situation relates to.
A macrodiversity connection means a situation where a cellular radio system terminal is simultaneously in radio connection with at least two base stations, whereupon the same information can be routed from the terminal to the network or from the network to the terminal through at least two different routes. In particular, a macrodiversity connection can be utilised in systems based on spread spectrum technique when the terminal is close to the boundary between cells or in an area where several cells are located entirely or partly on top of one another. A procedure, where a terminal drawing away from a given first base station first establishes a macrodiversity connection wherein it is simultaneously in communication with the first and a second base station, is called a soft handover. The terminal will transfer completely under the second base station only after the connection through the second base station becomes preferable to the macrodiversity connection. Drawing away should be understood in a broad sense, i.e. so that the connection to the first base station becomes poor in relation to the quality target set on the connection either as the physical distance grows, interference impeding the connection increases or the connection quality target changes.
In a cellular radio system based on spread spectrum technique, it is preferable for the performance of the system to keep the transmission powers as low as possible in both terminals and base stations. In a macrodiversity connection, it is possible to use a lower transmission power than if the connection between the terminal and the network went through only one base station while the other factors remain unchanged. On the other hand, spread spectrum technique provides naturally good opportunities for connecting such signal components, which arrive at a combining point at different power levels and delays either because of different types of propagation paths on the radio path or due to macrodiversity. Because of these factors, more and more macrodiversity connections will probably be used in the future. The commonest application of spread spectrum technique is the CDMA (Code Division Multiple Access) cellular radio system.
FIG. 1 shows a well-known situation where a terminal (MS, Mobile Station) 100 is simultaneously in radio connection with base stations (BS) 101 and 102. What is particular in the case shown in FIG. 1 is that the BS 101 operates under a first radio network controller (RNC) 103 and the BS 102 operates under a second radio network controller 104. An interface 105 between a RNC and a base station is called an lubis interface and an interface 107 between a RNC and a core network 106 (CN) is called an lu interface. The abbreviation or the part lu of the abbreviation comes from the words Interface UMTS where UMTS means a proposal for a third generation digital cellular radio system (Universal System for Mobile Communications). An interface 108 between two RNCs is called an lur interface. It has been assumed in FIG. 1 that the RNC 103 is a so-called serving RNC of the macrodiversity connection shown in the figure, and the RNC 104 is a so-called drift RNC subordinated thereto. The combining of signal components essential for the macrodiversity connection takes place in the serving radio RNC 103 according to the definition. A part 109 of the RNC wherein the combining takes place is called a MDC (MacroDiversity Combiner).
In a macrodiversity connection, a signal's paths between the terminal 100 and the combiner 109 are called branches. Due to macrodiversity, it is possible to use in each branch lower transmission power than if the corresponding branch established a single connection between the terminal and the network. Also the combined power of the branches remains lower than in a conventional single connection. In FIG. 1, the macrodiversity connection consists of three branches two of which go between the serving RNC and the terminal directly through the serving BS and one branch goes through the drift RNC and the drift BS.
Each RNC is responsible for the so-called network balancing in the area of its own base stations. In practice, this means that the RNC sets upper and lower limits on the number of simultaneous connections, the amount of radio resources available for each connection and the transmission powers of the base stations and the terminals that are in radio connection with them so that the transmission powers are optimal as for the overall performance of the system. Network balancing is also called load control.
Each serving RNC is responsible for the call control of its own calls. Call control includes, e.g. combining new macrodiversity branches, removing existing macrodiversity branches or changing the connection parameters (e.g. data rate, transmission power or the spreading code to be used) of existing macrodiversity branches. Normally, so-called fast closed-loop power control where transmission power control is based on measuring the ratio between the signal power received at the base station and the interference power and comparing the measurement result to the set target value, is applied in each radio connection. The information describing the comparison result is transmitted as response to the transmitting device. Another part of the power control is the outer power control loop which, at regular intervals, computes a new target value for the closed-loop power control on the basis of the quality of the connection (e.g. bit error ratio) at the time in question. Because the serving RNC's MDC is the only place where the combined quality of the signal components, arriving along different paths, of a terminal in a macrodiversity connection can be measured, the serving RNC's call control computes the closed-loop power control target values.
In a situation shown in FIG. 1, the problem is that the connection-parameters controlled by the call control contained in the serving RNC can be contradictory to the limitations set by the drift RNC's load control either because the connection parameters change during the connection or because the limitations change during the connection.